1. Technical Field of the Invention
The present invention relates to a process of hydrocyanation of ethylenically unsaturated organic compounds to compounds having at least one nitrile function.
More particularly, it relates to the hydrocyanation of diolefins such as butadiene, or of substituted olefins such as alkene nitrites such as pentenenitriles.
2. Description of the Prior Art
French Patent No. 1,599,761 describes a process of preparation of nitrites by the addition of hydrocyanic acid to organic compounds having at least one ethylenic double bond, in the presence of a catalyst of nickel and a triaryl phosphite. This reaction can be performed with or without the presence of a solvent.
When a solvent is used in this prior art process, it is preferably a hydrocarbon such as benzene or the xylenes or a nitrile such as acetonitrile.
The catalyst used is an organic nickel complex containing ligands such as phosphines, arsines, stibines, phosphites, arsenites, or antimonites. The presence of a promoter to activate the catalyst, such as a boron compound or a metallic salt, generally a Lewis acid, is likewise already recommended in the said patent.
Much work has been carried out to find catalytic systems generally comprising an organophosphorus ligand and a catalytically active metal, more particularly nickel, and exhibiting higher and higher performances.
The performance of a catalytic system is evaluated by determining several characteristics such as, in particular, the stability of the catalytic activity of the system, the yield of the reaction, and the selectivity in the synthesis of advantageous products: in the present case, the selectivity for linear pentenenitriles or for adiponitrile.
Thus it was proposed at an earlier time to use as a catalytic system, nickel associated with organophosphorus ligands possessing a single phosphorus atom, termed monodentate ligands. The industrially used compound of this class of products is tritolyl phosphite (TTP). This system has very acceptable performance in the synthesis of pentenenitriles by the hydrocyanation of butadiene, but a performance in need of improvement for the synthesis of adiponitrile by hydrocyanation of pentenenitriles.
Furthermore, this catalytic system has good stability and solubility in the reaction medium. Such catalytic systems have been described in numerous patents such as, for example, U.S. Pat. No. 3,496,215, DE 19953058, FR 1,529,134, FR 2,069,411, U.S. Pat. Nos. 3,631,191 and 3,766,231, FR 2,523,974.
To obtain catalytic performance, in particular high selectivity in the step of hydrocyanation of alkene nitriles, more particularly pentenenitriles, to dinitriles, a new class of organophosphorus ligands has been proposed, specifically intended to be associated with nickel. This new class of ligands comprises organophosphorus compounds possessing several phosphorus atoms, termed pluridentate ligands. Among these, the proposed compounds are generally compounds including two phosphorus atoms, termed bidentate ligands.
Such compounds and catalytic systems are protected by numerous patents. By way of example, there can be mentioned WO 99/06355, WO 99/06356, WO 99/06357, WO 99/06358, WO 99/52632, WO 99/65506, WO 99/62855, U.S. Pat. No. 5,693,843, WO 96/1182, WO 96/22968, U.S. Pat. No. 5,981,772, WO 01/36429, WO 99/64155, and WO 02/13964.
The structure of these compounds is more or less complex, particularly as regards the groups carried by the phosphorus atoms and the structure connecting together the two phosphorus atoms.
With this class of bidentate ligands, it is possible to obtain a catalytic system having in particular a better selectivity in the production of linear dinitriles in the process of hydrocyanation of an alkene nitrile.
However, these bidentate ligands of more complex structure are more difficult to synthesize, making their cost higher. Consequently, it is necessary and important that their stability in the reaction medium is very high to permit utilization in an industrial process from an economic viewpoint. Furthermore, taking account of their complex structure, their solubility in the reaction medium can be reduced and can lead to a lowering of catalytic activity and thus of the total yield of the hydrocyanation reaction.